TM 9-2350-247-10OPERATE NBC AIR FILTER SYSTEM (M548A3)  Continued 0053 003. Turn heater control knob left (counter-clockwise) to OFF position.4. Disconnect air hose from canister of M25A1 protective mask.5. Install air hose assembly on orifice connector assembly.NOTETypically, the M548A3 will have only four NBC stations. However, M548A3 vehicles convertedfrom M730 vehicles may have five NBC stations.6. Pull protective mask microphone connector from receptacle on helmet and plug in boom microphone connector toreceptacle on helmet.0053 00-8